The hour is late and you sit down for dinner with a friend. Both of you are hungry. You demolish your starter along with a few of chunks of bread and then dig into your main course. Your partner begins to slow down, but you plough on. Your eat everything on your plate and then order a dessert. Only after you have scoffed that do you stop, aware that you have lost your appetite and at last feel full. If nothing else, your performance explains why you are so much rounder and heavier than your companion.
Such diversity in eating habits reveals a simple culinary truth: that some people find it easier to stop eating than others. Somehow the chemicals in their digestive systems react more quickly to their food than other people's and so quickly switch off their hunger pangs.
The exact mechanisms involved in humans' response to food have baffled scientists for decades. But recently a series of breakthroughs has raised hopes it will soon be possible to interfere with this process and create drugs to suppress our appetites and control our weights. Appetite suppressants based on hormones that are produced in our gut after we have eaten are already producing encouraging results.
"Humans evolved at times when food was scarce and when we faced starvation all the time," says Professor Waljit Dhillo, at Imperial College, London. "Those who survived were the ones who were able to eat most food and could sustain themselves through periods of famine. They passed on the genes for that ability to future generations. It was useful then, when times were hard, and it ensured our species survived famines.
"But now food is plentiful. We live in a world of McDonald's and Kentucky Fried Chicken, and that ability to gorge ourselves is proving harmful. Our genes keep telling us to eat more."
In 2009, 22% of men and 24% women in Britain were classed as being obese, while a total of 39% of adults had abnormal waist circumference compared with only 23% in 1993.
Even worse has been the impact on younger individuals. In 2009, 16% of boys aged two to 15 and 15% of girls were classed as obese, an increase from 11% and 12% respectively since 1995. Nor is there any sign that this swelling tide of numbers of overweight people is turning back.
"The crucial factor is that not everyone is badly affected by the problem," added Dhillo. "There is a range of different responses within the population. Finding out why some people respond well and others do not is proving to be crucial in developing potential appetite suppressors."
This point was backed by Dr Alasdair Mackenzie, of Aberdeen University. "It is a simple fact that some people crave food more than others. It is not addiction. It is just that some people, when they start eating, find it a lot easier to stop. Finding what is going on, and uncovering the precise biochemical mechanisms involved, is proving to be highly enlightening."
In the past, scientists assumed that the hormones produced in the gut during digestion were involved in breaking down food.
But in the past decade they have discovered that many of these chemicals play a very different role. They are messengers which tell the brain that it is time to halt taking in food as enough has now been consumed.
"There appears to be many chemicals involved in sending chemical messengers from the gut to the brain," added Dhillo. "The trick is to find the most important ones."
A region of the brain called the hypothalamus plays a critical role in dealing with digestion and controls chemical messengers that pass between the brain and the gut. Researchers have isolated two of these messengers, chemicals released by cells in the intestine which are closely linked to appetite suppression. One is known as Glucagon-like peptide-1, or GLP-1, and the second is known as Peptide YY, or PYY. "We recently carried out experiments on adults who had fasted for 12 hours," added Dhillo. "We monitored their brain activity and found, when we showed them pictures of food, particular areas of their brains lit up in our scanners. It was a measure of how interested they were in food.
"Then we gave them an infusion of GLP-1 and PYY and again showed them the pictures of food. Their brains did not light up nearly so much. In other words, they were less stimulated by the sight of food. They had lost their hunger. Essentially, PYY and GLP-1 suppressed their appetites."
Both hormones break down easily in the gut and so their direct use as a drug is limited. However, chemically altered versions of GLP1 – known as exenatide and liraglutide – are already used as a treatment for diabetes because they cause weight loss and also boost insulin secretion in the body. "The trouble with these drug hormones is that they are short-acting," added Dhillo. "Most break down quickly. So pharmaceutical companies are developing analogue versions which have the same effect but last for longer in the body. Some could be administered once a week.
"The future will be to develop a multi-hormone injection that is given in a low dose, so that you do not get any side-effects, and will be long-acting enough so that you need only take it once a day or once a week.
"You will wake up hungry, but because you have had an injection of gut hormone you will only eat half what you would otherwise consume because your appetite has been suppressed.
"Unlike the current suppressors that are on the market, these gut hormones should have very few side-effects. They are versions of chemicals our bodies produce naturally after every meal we eat. That is another important point."
At present scientists are only at the stage of carrying out proof-of-principle studies. Nevertheless, Dhillo said he was optimistic of ultimate success. This hope was also backed by Mackenzie. His work has focused on a different messenger, a chemical known as galanin. Its output in the body is controlled by the hypothalamus. When it is released, it stimulates a desire to eat fatty foods and consume alcohol.
"We have found that there are two versions of this galanin switch. A person could have a weaker one that produces relatively low amounts and so triggers only a mild urge to consume fatty foods or alcohol. Alternatively a person might have the strong version which triggers a high output of galanin and stimulates a strong urge to eat fatty foods."
Intriguingly, Mackenzie has found that Europeans are more likely to carry strong versions of the galanin switch than Asian populations, so Europeans will have, on average, a greater urge to eat fatty foods and to drink alcohol.
"We are now trying to find the chemical pathways in the brain that control that weak-strong switch. We could then manipulate that pathway with drugs, so that we could turn a strong switch into a weak switch and so turn down an individual's interest in eating fatty foods. In that way we could begin to suppress people's appetite and reduce their urges to eat fatty foods."
In short, we are not yet at the point where we can make a pill that we will be able to take when we want to stop feeling hungry. But we may not be far off.
OBESITY IN THE UK
About 70% of men and 63% of women in the UK are overweight or obese.
The number of obese people in the UK has tripled over the last 20 years. By 2020 at least one-third of adults, one-fifth of boys and one- third of girls will be obese.
One in five children in Britain eats no fruit at all.
In 2008 the amount of fruit and vegetables British people bought fell. Purchases of fresh fruit fell by 7.7% between 2007 and 2008 and the volume of fresh vegetables bought fell by 9.6%.
In 2009 about one in 10 schoolchildren aged between four and five was classified as obese.
Obesity reduces life expectancy by an average of nine years.
The body mass index (BMI) – your weight in kilograms divided by the square of your height in metres – is used to define obesity. Obese people have a BMI reading of more than 30. A person with a BMI of between 25 and 29.9 is considered overweight.
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